Semiconductor device having stiffener

ABSTRACT

A semiconductor device includes a substrate, a semiconductor element mounted on the substrate and a stiffener attached via an adhesive to a surface of the substrate opposite to a surface thereof on which the semiconductor element is mounted. The adhesive has a coefficient of thermal expansion smaller than that of the substrate and that of the stiffener, and the modulus of longitudinal elasticity of the adhesive is equal to or larger than 10 GPa. Otherwise, the adhesive has coefficient of thermal expansion larger than that of the substrate and that of the stiffener, and the modulus of longitudinal elasticity of the adhesive is equal to or smaller than 10 GPa. The height of the stiffener is less than that of the external terminals.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device provided with astiffener.

2. Description of the Related Art

In a semiconductor device having a semiconductor element mounted on asubstrate, a problem arises in that a warp may occur in the substrateand in the semiconductor element due to difference between coefficientof thermal expansion the semiconductor element and that of thesubstrate. Accordingly, there have been proposals wherein a stiffener (areinforcement member) is attached to the surface of the substrateopposite to that mounting the semiconductor element thereon to preventthe substrate and the semiconductor element from warping (for example,see Japanese Unexamined Patent Publication No. 3-295263, No. 6-204656,Japanese Unexamined Patent Publication No. 6-4579, No. 8-153938, No.6-82839, No. 11-260953, No. 2000-174176, No. 2001-101375, No.2001-15561, No. 2002-231769, and No. 2002-252301).

As the rigidity of the substrate increases if a metallic stiffener isattached to the substrate, the substrate is resistant against thebending to minimize the warp of the substrate and of the semiconductorelement. In this case, if coefficient of thermal expansion of thestiffener is made to be the same level as that of the substrate, thewarp of the substrate and the semiconductor element can be even morereduced. However, as the warp of the substrate and that of thesemiconductor element are not completely eliminated, there is still arequirement for further reducing the warp of the substrate and of thesemiconductor element.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a semiconductor devicecapable of further reducing the warp of the substrate and that of thesemiconductor element.

In one aspect of the invention, a semiconductor device, according to thepresent invention, comprises a substrate, a semiconductor elementmounted on the substrate and a stiffener attached via an adhesive to asurface of the substrate opposite to a surface thereof on which thesemiconductor element is mounted, wherein coefficient of thermalexpansion of the adhesive is smaller than that of the substrate and thatof the stiffener, and modulus of longitudinal elasticity of the adhesiveis equal to or larger than 10 GPa.

In another aspect of the invention, a semiconductor device, according tothe present invention, comprises a substrate, a semiconductor elementmounted on the substrate and a stiffener attached via an adhesive to asurface of the substrate opposite to a surface thereof on which thesemiconductor element is mounted, wherein coefficient of thermalexpansion of the adhesive is larger than that of the substrate and thatof the stiffener, and modulus of longitudinal elasticity of the adhesiveis equal to or smaller than 10 GPa.

In a further aspect of the invention, a semiconductor device, accordingto the present invention, comprises a substrate, a semiconductor elementmounted on the substrate and a stiffener attached via an adhesive to asurface of the substrate opposite to a surface thereof on which thesemiconductor element is mounted, wherein the stiffener is a frame-likemember having an outer contour generally identical to that of thesemiconductor element, and external terminals are provided around thestiffener, a height of the stiffener being smaller than that of theexternal terminals.

As a result, the inventors of the present invention have studied thewarp of the substrate and the warp of the semiconductor element in thesemiconductor device provided with the stiffener, and it has been foundthat the warp of the substrate and the warp of the semiconductor elementcan be avoided by suitably selecting not only the characteristics of thestiffener but also the characteristics of the adhesive used for fixingthe stiffener to the substrate. Accordingly, it is possible to furtherminimize the warp of the substrate and the warp of the semiconductorelement if the characteristics of the material forming the respectivemembers are properly selected to satisfy the above-mentionedrelationship of between the coefficient of thermal expansion of thesubstrate and that of the stiffener and that of the adhesive, and themodulus of longitudinal elasticity of the adhesive.

Also, in the case of the semiconductor device having external terminalsprovided on the substrate around the stiffener, if a height of thestiffener is less than that of the external terminals, the externalterminals formed as solder balls are assuredly bonded to the terminalsof the printed circuit board and the stiffener is brought into contactwith the surface of the printed circuit board, whereby the stiffenerexhibits a stand-off function to properly connect the substrate to theprinted circuit board. Thus, the warp of the substrate and the warp ofthe semiconductor element is further minimized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a semiconductor device according tothe present invention;

FIG. 2 is a bottom view of the semiconductor device shown in FIG. 1;

FIG. 3 is a view illustrating the warp of the semiconductor elementrelative to the modulus of longitudinal elasticity of the adhesive whenthe stiffener is made of stainless steel;

FIG. 4 is a view illustrating the warp of the semiconductor elementrelative to the modulus of longitudinal elasticity of the adhesive whenthe stiffener is made of copper;

FIG. 5 is a view illustrating the warp of the semiconductor elementrelative to the modulus of longitudinal elasticity of the adhesive whenthe stiffener is made of stainless steel;

FIG. 6 is a view illustrating the warp of the semiconductor elementrelative to the modulus of longitudinal elasticity of the adhesive whenthe stiffener is made of copper;

FIG. 7 is a view illustrating an example in which the semiconductordevice shown in FIG. 1 is being mounted on a printed circuit board;

FIG. 8 is a view illustrating the semiconductor device of FIG. 7 afterit is mounted; and

FIG. 9 is a view illustrating an example of the semiconductor deviceshown in FIG. 7 to which a heat sink is attached.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention will now be describedwith reference to the attached drawings. FIG. 1 is a cross-sectionalview of the semiconductor device according to the present invention andFIG. 2 is a bottom view of the semiconductor device shown in FIG. 1.

In FIGS. 1 and 2, the semiconductor device 10 includes a substrate 12, asemiconductor element (an LSI chip) 14 mounted on the substrate 12, anda stiffener 16 attached via an adhesive 18 to a surface of the substrate12 opposite to that carrying the semiconductor element 14 thereon. Bumps(solder balls) 20 are provided on electrode pads of the semiconductorelement 14, and joined to corresponding electrode pads of the substrate12 by a flip-chip bonding. An under-fill material 22 is filled aroundthe bumps 20 in a gap between the semiconductor element 14 and thesubstrate 12.

The stiffener 16 is a frame-like member having an outer contourgenerally identical to that of the semiconductor element. The stiffener16 is attached to the substrate 12 so that the center of the stiffener16 coincides with the center of the semiconductor element 14. Further,external terminals 24 comprising solder balls are provided on thesubstrate 12 around the stiffener 16, and electric parts 26 such ascapacitors or others are provided on the substrate 12 within thestiffener 16. Circuits are formed in the substrate 12 and the externalterminals 24 are connected to electrode pads on the bumps 20 of thesubstrate 12 via the circuit. The external terminals 24 are arranged ina BGA.

The height of the stiffener 16 is less than the height of the externalterminals 24. For example, the thickness of the stiffener 16 is 0.4 mm,and the diameter of the solder ball forming the external terminals 24 is0.6 mm.

The stiffener 16 is made of a metallic material having coefficient ofthermal expansion approximately the same as that of the substrate 16.For example, the stiffener 16 is made of stainless steel (SUS 304)having coefficient of thermal expansion of 17.3 ppm/° C. and modulus oflongitudinal elasticity of 5 GPa. The stiffener 16 may be made of othermetallic material such as copper. The coefficient of thermal expansionof copper is 17 ppm/° C. The substrate 12 is made of organic resin suchas BT resin and coefficient of thermal expansion of the substrate 12containing circuit materials is 17 ppm/° C.

The adhesive 18 is made of a material having a modulus of longitudinalelasticity determined in accordance with the relationship betweencoefficient of thermal expansion of the adhesive 18, and those of thesubstrate 12 and the stiffener 16. That is, (a) when coefficient ofthermal expansion of the adhesive 18 is smaller than those of thesubstrate 12 and the stiffener 16, the modulus of longitudinalelasticity of the adhesive is equal to or larger than 10 GPa, and (b)when coefficient of thermal expansion of the adhesive 18 is larger thanthose of the substrate 12 and the stiffener 16, the modulus oflongitudinal elasticity of the adhesive is equal to or smaller than 10GPa.

FIG. 3 is a view illustrating the warp of the semiconductor element (LSIchip) 14 relative to modulus of longitudinal elasticity of the adhesive18 when the stiffener 16 is made of stainless steel. The amount of warpis represented in mm when the semiconductor element 14 is of 20 mmsquare (i.e., mm/20 mm square). The curve A is obtained when coefficientof thermal expansion of the adhesive 18 is 30 ppm/° C.; the curve B isobtained when coefficient of thermal expansion of the adhesive 18 is 25ppm/° C.; and the curve C is obtained when coefficient of thermalexpansion of the adhesive 18 is 20 ppm/° C. These values (30, 25 and 20)of coefficient of thermal expansion are larger than coefficient ofthermal expansion (17) of the substrate 12 and that (17.3) of thestiffener 16. In this case, if modulus of longitudinal elasticity isequal to smaller than 10 GPa, the amount of warp of the substrate 12becomes smaller. If modulus of longitudinal elasticity is equal to orlarger than 10 GPa, the amount of warp of the substrate 12 graduallyincreases. For example, if coefficient of thermal expansion of theadhesive 18 is 40 ppm/° C., the amount of warp of the substrate 12becomes larger than the values on the curve A in FIG. 3 in the range ofmodulus of longitudinal elasticity of the adhesive 18 larger than 10GPa, but the amount of warp of the substrate 12 is approximately thesame as the values on the curve A of FIG. 3 in the range of modulus oflongitudinal elasticity of the adhesive 18 is equal to or smaller than10 GPa. Accordingly, it is possible to use the adhesive 18 havingcoefficient of thermal expansion of 40 ppm/° C.

FIG. 4 is a view illustrating the warp of the semiconductor element (LSIchip) 14 relative to modulus of longitudinal elasticity of the adhesive18 when the stiffener 16 is made of copper. The curve D is obtained whencoefficient of thermal expansion of the adhesive 18 is 30 ppm/° C.; thecurve E is obtained when coefficient of thermal expansion of theadhesive 18 is 25 ppm/° C.; and the curve F is obtained when coefficientof thermal expansion of the adhesive 18 is 20 ppm/° C. These values (30,25 and 20) of coefficient of thermal expansion are larger thancoefficient of thermal expansion (17) of the substrate 12 and that (17)of the stiffener 16. In this case, if modulus of longitudinal elasticityof the adhesive 18 is equal to or smaller than 10 GPa, the amount ofwarp of the substrate 12 is small. Contrarily, if modulus oflongitudinal elasticity is larger than 10 GPa, the amount of warp of thesubstrate 12 gradually increases.

FIG. 5 is a view illustrating the warp of the semiconductor element (LSIchip) 14 relative to modulus of longitudinal elasticity of the adhesive18 when the stiffener 16 is made of stainless steel. The curve G isobtained when coefficient of thermal expansion of the adhesive 18 is 15ppm/° C.; the curve H is obtained when coefficient of thermal expansionof the adhesive 18 is 10 ppm/° C.; and the curve I is obtained whencoefficient of thermal expansion of the adhesive 18 is 5 ppm/° C. Thesevalues (15, 10 and 5) of coefficient of thermal expansion are largerthan coefficient of thermal expansion (7) the substrate 12 (17) and that(17.3) of the stiffener 16. In this case, if modulus of longitudinalelasticity of the adhesive 18 is equal to or larger than 10 GPa, theamount of warp of the substrate 12 becomes considerably small.

FIG. 6 is a view illustrating the warp of the semiconductor element (LSIchip) 14 relative to the modulus of longitudinal elasticity of theadhesive 18 when the stiffener 16 is made of copper. The curve J isobtained when coefficient of thermal expansion of the adhesive 18 is 15ppm/° C.; the curve K is obtained when coefficient of thermal expansionof the adhesive 18 is 10 ppm/° C.; and the curve L is obtained whencoefficient of thermal expansion of the adhesive 18 is 5 ppm/° C. Thesevalues (15, 10 and 5) of the coefficient of thermal expansion are largerthan coefficient of thermal expansion (17) of the substrate 12 and that(17.3) of the stiffener 16. In this case, if modulus of longitudinalelasticity of the adhesive 18 is equal to or larger than 10 GPa, theamount of warp of the substrate 12 becomes considerably small.

The adhesive 18 is a thermosetting resin formed of an adhesive resinmaterial (such as epoxy resin) containing fillers and is provided as anadhesive sheet. Coefficient of thermal expansion and modulus oflongitudinal elasticity of the adhesive 18 are measured after theadhesive sheet is heated and cured. The adhesive 18 has a thickness inthe range from 50 to 100 μm, but the thickness of the adhesive 18 has noinfluence on the amount of warp of the substrate 12.

The above-mentioned result is obtained when the stiffener 16 and thesubstrate 12 have substantially the same coefficients of thermalexpansion. If the coefficient of thermal expansion of the stiffener 16is considerably different from that of the substrate 12, the result maybe distinct. For example, when the stiffener 16 is made of aluminum(having coefficient of thermal expansion of approximately 23), theamount of warp of the substrate 12 is approximately 0.14, which isconsiderably larger than the values shown in FIGS. 3 to 8 and isfavorable.

FIG. 7 is a view illustrating an example in which the semiconductordevice 10 shown in FIG. 1 is being mounted on a printed circuit board (amother board) 30. FIG. 8 is a view illustrating the semiconductor deviceof FIG. 7 after it is mounted. The printed circuit board 30 hasterminals 32 corresponding to the external terminals 24 of thesemiconductor device 10. As described above, the semiconductor device 10includes the substrate 12, the semiconductor element 14 mounted on thesubstrate 12, and the stiffener 16 attached via the adhesive 18 to asurface of the substrate 12 opposite to that carrying the semiconductorelement 14. The stiffener 16 is a frame-like member having an outercontour generally identical to that of the semiconductor element 14, andthe external terminals 24 are provided on the substrate 12 around thestiffener 16. A height of the stiffener 16 is smaller than that of theexternal terminal 24. After the semiconductor 10 is mounted on theprinted circuit board 30, the stiffener 16 is in contact with thesurface of the printed circuit board 30.

In the case of the semiconductor device 10 having the external terminals24 provided on the substrate 12 around the stiffener 16, the externalterminal 24 s, for example, formed as solder balls are securely bondedto the terminal 32 of the printed circuit board 30 and the stiffener 16is in contact with the surface of the printed circuit board 30, bysetting the height of the stiffener 16 smaller than that of the externalterminal 24, so that the stiffener 16 exhibits a stand-off function towell couple the substrate 12 to the printed circuit board 30, wherebythe warp of the substrate 12 and the semiconductor element 14 is furtherminimized.

FIG. 9 is a view illustrating an example of the semiconductor device 10shown in FIG. 7 to which a heat sink (cooling member) 34 is attached.The heat sink 34 has fins 36, and coupled to the semiconductor element14 via a heat conductive member 38. The heat conductive member 38 isformed of a good heat conductive material such as thermal compound orthermal sheet. The heat sink 34 is pressed onto the semiconductorelement 14 via the heat conductive member 38 and fixed to the printedcircuit board 30. In the example shown in FIG. 9, the printed circuitboard 30 is supported by a supporting plate 40, and screws 42 arethreaded in the supporting plate 40 through holes provided in the heatsink 34 and the printed circuit board 30. A coil spring 44 is locatedbetween the head of each screw 42 and the heat sink 34 to press the heatsink 34 onto the semiconductor element 14 due to an elastic force of thesprings 44.

In this way, by pressing the heat sink 34 onto the semiconductor element14 by the elastic force of the springs 44 to bring the stiffener 16 intocontact with the surface of the printed circuit board 30, the heatconductive member 38 is maintained at a constant thickness under thepredetermined pressure, and is capable of transferring heat from thesemiconductor element 14 to the heat sink 34 while stabilizing the heatresistance of the heat conductive member 38. Thus, the stiffener 16exhibits not only the above-mentioned warp reduction function and thestand-off function but also the stand-off function for avoiding anexcessive force applied to a portion between the external terminal 24and the terminal 32 due to the elastic force of the springs 44.Accordingly, a stress generated at the joint between the externalterminals 24 arranged as BGA and the terminals of the printed circuitboard 30 and in the semiconductor element 14 is mitigated to provide asemiconductor device high in reliability.

As describe above, according to the present invention, it is possible toobtain a highly reliable semiconductor device in which the warp of thesemiconductor element is small.

1. A semiconductor device comprising a substrate, a semiconductorelement mounted on the substrate and a stiffener attached via anadhesive to a surface of the substrate opposite to a surface thereof onwhich the semiconductor element is mounted, wherein the coefficient ofthermal expansion of the adhesive is smaller than that of the substrateand that of the stiffener, and the modulus of longitudinal elasticity ofthe adhesive is equal to or larger than 10 GPa.
 2. A semiconductordevice comprising a substrate, a semiconductor element mounted on thesubstrate and a stiffener attached via an adhesive to a surface of thesubstrate opposite to a surface thereof on which the semiconductorelement is mounted, wherein the coefficient of thermal expansion of theadhesive is larger than that of the substrate and that of the stiffener,and the modulus of longitudinal elasticity of the adhesive is equal toor smaller than 10 GPa.
 3. A semiconductor device comprising asubstrate, a semiconductor element mounted on the substrate and astiffener attached via an adhesive to a surface of the substrateopposite to a surface thereof on which the semiconductor element ismounted, wherein the stiffener is a frame-like member having an outercontour generally identical to that of the semiconductor element, andexternal terminals are provided around the stiffener, the height of thestiffener being smaller than that of the external terminals.
 4. Asemiconductor device as defined by claim 3, further comprising a printedcircuit board having terminals to be connected to the externalterminals, wherein the stiffener is brought in contact with a surface ofthe printed circuit board.
 5. A semiconductor device as defined by claim4, wherein a cooling member is coupled to the semiconductor element viaa heat-conductive member, and is fixed to the printed circuit board.